Gating currents of inactivating and non-inactivating potassium channels expressed in Xenopus oocytes

The Xenopus oocyte expression system in combination with patch-clamp techniques allows the measurement of ionic currents from a single class of genetically engineered ion channels. Ionic currents in the nanoampere range from oocytes injected with cRNA, corresponding to potassium channels, can be recorded in the inside-out patch configuration. These recordings have a high time resolution at low background noise. Substitution of impermeant ions for potassium and blocking of the channel conductance with tetraethylammonium allows the recording of potassium gating currents, Ig, which is hampered in natural excitable cells by the simultaneous presence of sodium channels and a variety of different potassium channels. The "on" transients, Ig(on), are fast and can have amplitudes of up to several tens of pA. Upon repolarization to -100 mV after small depolarizations, "off" gating currents, Ig(off)g, which reverse most of the "on" charge displacement, Q(on), within 1 ms, are readily observed. However, this fast recovery of the gating charge is drastically reduced upon increasing the amplitude of the depolarizing pulse. In contrast to sodium channels, this temporary charge immobilization is complete within a few milliseconds at positive membrane potentials. Furthermore, there seems to be no direct correlation between charge immobilization and inactivation because the same phenomenon occurs for channels that do not inactivate.